Prior methods of determining the fission flux within the test specimen generally requires a detailed knowledge of the fuel region properties. This knowledge allows a determination of the conductivity, k, as a function of temperature. The French scientist J. B. J. Fourier in 1822 proposed in Cartesian coordinates the one-dimensional heat transfer equation q=-kA dT/dx, where q=heat rate, T=temperature, A=area, and dx=incremental thickness. This equation in most simple applications leads to .intg.kdT=S, where k must be known as a function of T to solve this equation. While these previous methods depend strongly on stated fuel properties, the method of the invention, as described below, relies more on experimental data, and the assumption of a similar thermal conductivity temperature variation.